1. Field of the Invention
The present invention relates to a stationary-original-table type image reading apparatus using a contact type image sensor.
2. Related Background Art
In an image reading apparatus, a charge-coupled device (CCD) image sensor or a contact image sensor or a contact image sensor (hereinafter referred to as CIS) is used. As compared with the CCD image sensor, the CIS has many advantages. For example, it allows a reduction in apparatus size, and its driving system is substantially simplified, so that there is practically no need for adjustment. Further, no image distortion due to the reduction in optical system is generated.
On the other hand, due to the construction of the CIS in which reflected light from the original surface is focused onto a photoreceptor by means of a SELFOC lens array, the CIS has a problem in that the focal depth is rather small and thus the image is likely to be out of focus unless the support structure for the CIS has sufficient accuracy. In view of this problem, Japanese Patent Application Laid-Open No. 07-193689 and U.S. Pat. No. 5,801,851, etc. disclose a method according to which an image is read with the distance between the CIS and the original surface being kept constant by causing rollers, slide members or the like provided at the end portions of the upper surface of the CIS to abut the lower surface of the original support glass, whereby an out-of-focus state is prevented.
In a so-called book scan type image reading apparatus, which reads original image information with the original being fixed onto the original support glass, downward deflection of the original support glass can be generated when a thick original such as a book or a dictionary is placed on the original support glass and pressed by a presser plate. The deflection amount of the original support glass in the main scanning direction is maximum substantially in the central portion between the back end and the front end supports of the original support glass.
In all of the above-mentioned examples, the rollers, slide members or the like are caused to abut the lower surface of the original support glass outside the image reading range at the both ends of the CIS. Thus, of the image reading range of the CIS, the distance between the CIS and the original surface is maintained with accuracy near the both ends of the CIS, whereas in the central portion, an out-of-focus state is likely to be brought about when the original surface comes too close to the CIS beyond the optimum focal position.
FIG. 11 is a side sectional view of a conventional image reading apparatus. FIG. 12 shows how deflection is generated in an original support glass 3 by placing a thick original thereon and pressing it with a presser plate 8. The focal depth of the SELFOC lens mounted to a CIS 1 is ±0.5 mm, and a range of 1 mm across the optimum focal position is the effective focus range. The deflection amount of the original support glass 3 is maximum in the central portion between the end portions of the main scanning reading range of the CIS 1; in this case, it is assumed to be a downward deflection of 0.6 mm.
In the conventional image reading apparatus, abutment members 2 are arranged outside the main scanning reading range. Thus, as shown in FIG. 12, when the deflection amount of the original support glass 3 in the main scanning direction exceeds the effective focus range of the CIS 1, the portion of the upper surface of the original support glass 3 constituting the original surface is deviated from the effective focus range in the vicinity of the central portion of the main scanning reading range, so that an out-of-focus state is caused in this portion, resulting in a deterioration in image quality.